Abstract

The subject of this dissertation is the interaction of jets in radio galaxies and quasars with their environments. When radio jets expand from the nucleus of an active galaxy, they pass through the interstellar medium of the host galaxy and the neighbouring intergalactic medium. The pressure forces acting on the jet bend it and induce a curvature giving rise to a non-straight shape of the jet. Another interaction occurs when expanding jets collide with stratified clouds in the interstellar and intergalactic medium.

To model deflections produced by jet-cloud and jet-galaxy interactions, a model was developed based on the assumption that, for a jet which expands adiabatically and has reached a steady state, pressure balance must be maintained between the jet material and its surrounding environment. The main consequence of the model is that the bendings in the jets are very sensitive to their initial velocities. For instance, a jet with a high initial Mach number will penetrate the stratified density region with an almost straight trajectory, whereas a low Mach number jet will show pronounced curvatures.

By studying the characteristics emanating from the a jet that bends, it is possible to set an upper limit to the maximum bending angle (the angle the jet makes with its original straight trajectory) for which a jet will not produce an internal shock. The result is that non-relativistic jets with a classical equation of state can only bend by no more than $\sim \unit{ 75 }{ \degree }$ whereas relativistic jets cannot be deflected more than $\sim \unit{ 50 }{ \degree }$.

An important interaction between jets and their surrounding environment occurs in some radio galaxies which show alignment between their optical and radio emission, the so called radio/optical alignment effect. This radiation has been shown to be produced by shock waves in small radio sources. It appears that cold clouds embedded in the interstellar and intergalactic medium collide with the expanding jet, producing shocks which are able to induce the observed emission. When the jets expand, they collide with cold clouds giving rise to a natural scenario for which shock waves are produced as a result of the collision. A model for this shock/cloud collision in one dimension has been developed. This shows in detail that the interaction of the bow shock of the jet transmits a shock inside the cloud and reflects back a rarefaction wave once it crosses it and collides with the rear of the cloud.